The present invention relates to a system for optimizing operation of lean bum internal combustion (IC) engines for achieving the best trade-off between exhaust emissions, fuel economy, and engine power. The invention includes: 1) a strategy or process, 2) a general apparatus, and 3) a range of preferred embodiments for combining engine air, exhaust gas, and fuel in relation to engine intake air manifold absolute pressure (MAP) for achieving optimization of exhaust emissions, efficiency, and power. The invention preferably employs recently improved forms of very high power, very high energy (VHE) ignition disclosed in several prior patents and patent applications, including U.S. Pat. No. 4,677,960 on voltage doubling, U.S. Pat. No. 4,774,914 on piston firing, U.S. Pat. No. 4,841,925on enhanced toroidal gap ignition, U.S. patent application Ser. No. 07-350,945 (now abandoned) on high efficiency and high output coils, U.S. patent application Ser. No. 07-684595 now U.S. Pat. No. 5,131,376 on distributorless capacitive discharge ignition, and U.S. patent application Ser. No. 07-755,795 (now abandoned) on integrated converter ignition, which enhance the practicality of using the high air-fuel ratio and high exhaust dilution of the present invention for achieving said optimization. The present invention applies to all IC engines, and especially to four-stroke engines where NOx is relatively high and must be reduced and where fuel economy may be significantly improved over current approaches, i.e. the three-way catalyst approach and the lean bum approach.
In the three-way catalyst approach the air-fuel ratio is maintained at stoichiometry throughout the engine operating range by use of a lambda sensor. This results in relatively low NOx emissions but significantly compromised fuel economy (even when exhaust gas recirculation is employed via an EGR valve). In this approach engine-out (versus tailpipe) emissions are high, so that the system is not stable against malfunctions or even small departures from stoichiometric operation.
In modern commercial lean bum approaches the engine is operated at high air-fuel ratios (excess air) which improves fuel economy but does not reduce NOx to the required level, especially at high loads. Furthermore, modern commercial lean bum engines depend entirely on sophisticated engine design, i.e. intake air swirl and air tumble motion, to provide the very lean air-fuel ratio capabiity, and are thus not amenable for retrofit onto older cars (as in the case of the present invention). When further NOx reduction is required, they revert to the use of the conventional three-way catalyst approach with EGR, as described in SAE paper 920455, February 1992, by Honda Motors. Furthermore, the EGR valve adds the exhaust on the low pressure side of the throttle by means of engine vacuum and does not attempt to add the exhaust gas to a lean mixture in an optimized way as disclosed in the present invention.
Unconventional ways of operating lean burn engines, and advantages of doing so, are disclosed in my prior patent applications: U.S. patent application Ser. No. 07-685,057 (now U.S. Pat. No. 5,211,147) where reverse stratification is disclosed for reducing NOx, U.S. patent application Ser. No. 07/698,967 (now abandoned) where boosting of the intake air is disclosed for reducing NOx at high pressures, and U.S. patent applications 07/765,896 (now abandoned) and 07/854,074 (now abandoned) where methods of using exhaust gas in a lean burn mode are disclosed for reducing NOx emissions over a wide range of conditions.
General issues of engine emissions and fuel economy are discussed in many texts, and the following are a sampling of texts which discuss these issues in some degree: "Internal Combustion Engines and Air Pollution" by E. F. Obert (Intext Educational Publishers 1973); "Internal Combustion Engine Fundamentals" by John B. Heywood (McGraw-Hill Book Company, 1988); and the manual "Bosch Automotive Electric/Electronic Systems"(Robert Bosch GmbH 1988).
The present invention can be utilized with a variety of fuels including conventional petroleum-derived hydrocarbon mixture fuels, e.g., gasoline, or non-conventional petroleum and/or plant derived fuels, e.g., methanol, ethanol, natural gas, alcohol-hydrocarbon mixtures, etc. Discussion is limited to the conventional fuels, it being understood that the points discussed herein are applicable to all fuels with the appropriate correction factors well known to those skilled in the art.
Some terms used herein are now defined:
(1) Air-Fuel Ratio (AFR): The weight ratio of air to fuel (lbs to lbs or kilograms to kilograms) as the vapor form equivalent of given weights of air and fuel at standard temperature and pressure (STP) in accordance with standard industry practice which takes AFR of 14.7 to 1 (14.7:) as the stoichiometric ratio (14.7 lbs of air combusting 1 lb of gasoline). An alternate expression of AFR is lambda (.lambda.), wherein lambda equals AFR/(AFR)stoichiometry, or 1.0 at stoichiometric mix and STP conditions. AFR varies in accord with certain engine settings, principally engine speed and manifold absolute pressure (MAP). PA1 (2) Gas-fuel ratio (GFR): As used herein, is the same as air-fuel ratio excepting that the component that comprises air in this case includes exhaust gas, i.e. the "gas" comprises a combination of fresh intake air and exhaust gas. PA1 (3) Exhaust-fuel ratio (EFR): As used herein is defined as the difference between gas-fuel ratio and air-fuel ratio, i.e. EFR=GFR-AFR, and represents the ratio of the amount of exhaust gas introduced into the intake stream to the amount of fuel introduced. PA1 (4) Excess air-fuel ratio (EAFR): As used herein is defined as the difference between air-fuel ratio and stoichiometry, and represents the ratio of the amount of excess air introduced into the intake stream to the amount of fuel introduced. PA1 (5) Lean-Burn (or Lean of Stoichiometric, or dilute charge): Operation of an IC engine at AFR above stoichiometric, i.e. at or above 15:1 AFR for lean burn gasoline engines. PA1 (6) Three way catalyst and Lambda closed loop control: A multi-mode (three way catalyst) oxidation-reduction exhaust clean-up system which is used in a major portion of the world's automotive engines. A closed loop feedback maintaining lambda close to one is employed in state of the art systems. PA1 (7) Exhaust Gas Recirculation (EGR): The process of exhaust or burnt gas (as a percentage) recirculation from the exhaust into the unburnt gas (the fuel-air mixture) at the low pressure side of the throttle, the recirculation occurring at most engine operating conditions but specifically excluding idle and WOT. PA1 (8) Residual Gas or Residual Gas Fraction or Residual: The amount (fraction) of exhaust gas remaining in the inducted fuel-air mixture after the exhaust valve closes to participate in the combustion of the inducted mixture. PA1 (9) Exhaust Gas Dilution (EGD): As used herein is the process of diluting the intake air with substantial amount of exhaust gas at the high pressure side of the throttle including at near WOT conditions, under conditions of lean burn so that the excess oxygen can be used to oxidize HC and CO without external injected air. PA1 (10) Stratified Charge: Generally defined to mean the purposeful formation of a non-uniform fuel-air mixture or charge in the engine cylinder prior to combustion, where a locally richer mixture is produced at the spark plug site so as to help ignition of an overall leaner mixture. PA1 (11) Ignition and Fuel Injection Timing: The degrees before top dead center (BTDC), or more simply before top center (BTC), of piston stroke (or rotary engine equivalent) where ignition and fuel injection respectively commence. Emissions and specific fuel efficiency as a function of advanced or retarded ignition timing have been substantially studied in the art. PA1 (12) Valve Timing: The degrees, before and after top center (BTC and ATC) where the intake valve opens and the exhaust valve closes, and the degrees before and after bottom center (BBC and ABC) where the exhaust valve opens and the intake valve closes. PA1 (13) Wide Open Throttle (WOT): The operating condition of an engine in which the throttle or other means controlling air-flow into the cylinder is opened to permit essentially the maximum amount of air to enter the cylinder. PA1 (14) Manifold Absolute Pressure (MAP): The absolute pressure, typically in units of atmospheres, inside the intake manifold of an IC engine beyond the throttle plate, representing the pressure of the air which is inducted into the engine cylinders. A MAP value of 1.0 represents a pressure of one atmosphere at standard temperature inside the engine intake manifold.
Other related terms are defined below as used.
Primary interest in lean burn and stratified charge has come from developers of IC engines in specialty applications, e.g. stationary gas engines, and more recently to lean bum automobile engines to improve combustion (stability) and fuel economy, e.g. Toyota Motors, Honda Motors, and Mitsubishi Motors. Primary interest in catalysts and exhaust dilution have come from developers of three-way catalysts and EGR for use with such systems. These developers have failed to realize the potential benefits of properly prepared lean mixtures with high exhaust gas dilution. They have failed to successfully implement emerging technologies, especially very high energy (VILE) ignition, and have concentrated on a limited number of approaches based on three-way catalyst technologies and on lean burn technologies without exhaust dilution. For retrofit applications, three-way catalyst systems have not been implemented because of the need to include an air-pump and the requirement to limit, or even reduce, possible fuel economy gains achievable with the present system operated in a lean burn mode with exhaust gas dilution. Conventional lean burn has not been implemented because of the relatively high NOx levels.
In the above cited U.S. patent applications Ser. No. 07/765,896 (now abandoned) and 07/854,074 (now abandoned) are disclosed methods and apparatus for using principles of lean burn with high exhaust dilution to achieve significant improvements in emissions and fuel economy. The present invention builds on the ideas disclosed therein, including and not limited to the use of an exhaust gas dilution (EGD) tube with a control valve disclosed in the text of Ser. No. 07/765,896, and a strategy for use of exhaust dilution disclosed in the text of Ser. No. 07/854,074. Using this background, there is now disclosed a new principle or strategy, i.e. a process, for engine optimization, and there is further disclosed certain engine relationships which lead to a technique, i,e. a general apparatus, for realizing the optimization principle, and specific practical systems, i.e. preferred embodiments of process and apparatus for implementing the invention.